Calendar days and date offsets
Find the calendar-day gap between two dates and a separate date obtained by adding or subtracting days. Weekends remain part of the calendar count.
Understand the numbers · Make life easier
Period cycle dates
Summarize recorded period start dates, calendar gaps and their mean. Reference dates are arithmetic only, not ovulation, contraception or a medical prediction.
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Mean recorded gap
28.5days
The main answer is the mean recorded gap. Reference dates only add a whole-day average to the last start; they are not promised period dates or safe days.
Last start + mean gap2026-11-26
Gap count2
Min gap28 days
Max gap29 days
Last start2026-10-28
Last start + min gap2026-11-25
Last start + max gap2026-11-26
About the dataThese results describe your entries; future cycles may differ.
Worked steps
- 28 + 29 = 57
- 57 ÷ 2 = 28.5
Calculation details (2 rows)
| Start date | Next start date | Days between dates |
|---|---|---|
| 2026-09-01 | 2026-09-29 | 28 |
| 2026-09-29 | 2026-10-28 | 29 |
Each gap runs from one start date to the next without adding both endpoints; mean = total gaps divided by gap count. Reference dates add the mean rounded to whole days, minimum or maximum recorded gap to the last start. They are not prediction bounds or promised dates, may be in the past, and are not repeatedly advanced. Use 2 to 25 dates from 2000 to 2100. Gaps of 1 to 366 days are software limits, not normal ranges. This does not diagnose disease, confirm pregnancy or calculate ovulation, fertility or safe days, and must not be used for contraception. Seek medical care if concerned about persistent changes, pain or heavy bleeding. Inputs are calculated only in this browser.
Method and scope
A cycle gap runs from one period start to the next without adding both endpoints. The mean is the sum of gaps divided by their count.
Reference dates add the rounded mean, minimum or maximum observed gap to the last start. They are not prediction or confidence bounds, may already be in the past, and are not repeatedly advanced.
Use 2 to 25 dates from 2000 to 2100 and gaps of 1 to 366 days. These are software limits, not medical normal ranges. No diagnosis, pregnancy confirmation, ovulation, fertile window or contraception calculation is provided.
References:NHS: period start dates and recording cycles、U.S. Office on Women’s Health: cycle definition and variation
What this helps you do
Turn your recorded period start dates into a clear history of gaps, their mean, minimum and maximum. This is date arithmetic, not a diagnosis or a calculation of ovulation, pregnancy likelihood, safe days or contraception.
What to enter
Enter the first day of each separate period, not its end date or every day of bleeding. Use 2 to 25 dates from 2000 to 2100, oldest first, without duplicates. Choose dates individually or paste one YYYY-MM-DD per line. Missing records can make gaps longer, so check your original notes first.
How the calculation works
Each gap is the calendar-day difference from one start date to the next, without adding both endpoints. The mean is the total of the gaps divided by their count. A reference date adds the mean rounded to a whole day to the last start. The minimum and maximum gap dates use the corresponding historical gap lengths.
Worked example
For 2024-01-03, 2024-01-31 and 2024-03-01, the gaps are 28 and 30 days, giving a mean of 29 days. Adding 29 days to March 1 gives March 30. Adding the minimum and maximum gaps gives March 29 and March 31. These are arithmetic examples, not a promise that a period will occur within those dates.
Understanding the answer
The main answer is the mean gap between starts, not the number of bleeding days. The reference dates are not probability or confidence bounds. Two start dates provide only one observed gap, which cannot establish a long-term pattern. An old record can give a reference date already in the past; the tool does not repeatedly advance it.
Scope and limitations
The 1-to-366-day gap limit is a software boundary, not a medical normal range. Calendar arithmetic cannot verify whether an entry was a period, diagnose a condition or confirm pregnancy. Seek medical care if you are concerned about persistent changes, pain or heavy bleeding. Dates stay in this browser; keep shared-device data and backups private. Use the separate date-difference or pregnancy-date tool only for its own stated purpose.